The present invention is directed to a method and apparatus for forming a film through a transparent barrier in contact with a photohardenable composition in a manner that the film does not adhere to the barrier. It also pertains to a method and apparatus for fabricating an integral three-dimensional object from a multiplicity of cross sectional portions of the object through a transparent barrier in a manner that the cross sectional portions do not adhere to the barrier.
2. Description of Related Art
There are many systems known by which a film can be made. These include extrusion, drawing from a molten mass, casting, or spraying, or otherwise depositing a usually liquid film on a release substrate, solidifying the liquid film by decreasing the temperature, or by evaporating solvents contained in the film, or by other appropriate methods, and then separating the solidified film, and the like. The methods based on depositing a liquid film on a release substrate, and then separating the resulting solidified film are of particular interest because they require inexpensive equipment and they are simpler and easier to carry out. However, they have the disadvantage that the substrate cannot provide adequate release unless its surface tension is very low, which in turn makes difficult the film deposition without creation of defects, such as fish-eyes, craters, and the like, well known in the art of coatings and films.
In the area of Solid Imaging, many systems for production of three-dimensional modeling by photohardening have been proposed. European Patent Application No. 250,121 filed by Scitex Corporation Ltd., on Jun. 5, 1987, discloses a three- dimensional modeling apparatus using a solidifiable liquid, and provides a good summary of documents pertinent to this art U.S. Pat. No 4,575,330, issued to C. W. Hull on Mar. 11, 1986, describes a system for generating three-dimensional objects by creating a cross-sectional pattern of the object to be formed at a selected surface of a fluid medium capable of altering its physical state in response to appropriate synergistic stimulation by impinging radiation, particle bombardment or chemical reaction. Successive adjacent laminae, representing corresponding successive adjacent cross-sections of the object, are automatically formed and integrated together to provide a step-wise laminar buildup of the desired object, whereby a three-dimensional object is formed and drawn from a substantially planar surface of the fluid medium during the forming process U.S. Pat. No. 4,752,498, issued to E. V. Fudim on Jun. 21, 1988, describes an improved method of forming three-dimensional objects, which comprises irradiating an uncured photopolymer by transmitting an effective amount of photopolymer solidifying radiation through a radiation transmitting material which is in contact with the uncured liquid photopolymer. The transmitting material is a material which leaves the irradiated surface capable of further crosslinking so that when subsequent layer is formed it will adhere thereto. Using this method, multilayer objects can be made. U.S. Pat. No 4,801,477, issued also to Fudim on Jan. 31, 1989, mention is made of a light guide, which may be made of material containing copper, oxygen, or other ingredients that may inhibit photopolymer cross linking.
A publication entitled "Automatic Method for fabricating a three-dimensional plastic model with photohardening polymer" by Hideo Kodama, Rev. Sci. Instrum. 52(11), 1770-1773, Nov. 1981, describes a method for automatic fabrication of a three-dimensional plastic model. The solid model is fabricated by exposing liquid photohardening polymer to ultraviolet rays, and stacking the cross-sectional solidified layers. A publication entitled "Solid Object Generation" by Alan J Herbert, Journal of Applied Photographic Engineering, 8(4), 185-188, Aug. 1982, describes an apparatus which can produce a replica of a solid or three-dimensional object much as a photocopier is capable of performing the same task for a two-dimensional object. The apparatus is capable of generating, in photopolymer, simple three-dimensional objects from information stored in computer memory. A good review of the different methods is also given by a most recent publication entitled "A review of 3D Solid Object Generation" by A. J. Herbert, Journal of Imaging Technology 15: 186-190 (1989).
Most of these approaches relate to the formation of solid sectors of three-dimensional objects in steps by sequential irradiation of areas or volumes sought to be solidified. Various masking techniques are described as well as the use of direct laser writing, i.e., exposing a photohardenable composition with a laser beam according to a desired pattern and building a three-dimensional model, layer by layer. In addition to various exposure techniques, several methods of creating thin liquid layers are described which allow both coating a platform initially and coating successive layers previously exposed and solidified.
The aforementioned methods of coating, however, are not capable of ensuring flat uniform layer thickness or of producing such layers quickly, or they do not effectively prevent damage or distortion to previously formed layers during the successive coating process and they involve coating only liquid formulations of preferably low viscosity. Furthermore, they omit to recognize very important parameters involved in the coating process such as the effects of having both solid and liquid regions present during the formation of the thin liquid layers, the effects of fluid flow and rheological characteristics of the liquid, the tendency for thin photohardened layers to easily become distorted by fluid flow during coating, and the effects of weak forces such as hydrogen bonds and substantially stronger forces such as mechanical bonds and vacuum or pressure differential forces on those thin layers and on the part being formed.
The Hull patent, for example, describes a dipping process where a platform is dipped below the distance of one layer in a vat, then brought up to within one layer thickness of the surface of the photohardenable liquid. Hull further suggests that low viscosity liquids are preferable but, for other practical reasons, the photohardenable liquids are generally high viscosity liquids. Motion of the platform and parts, which have cantilevered or beam regions (unsupported in the Z direction by previous layer sections) within the liquid, creates deflections in the layers, contributing to a lack of tolerance in the finished part. In addition, this method is rather slow.
U.S. Pat. No. 2,775,758, issued to O. J. Munz on Dec. 25, 1956, and the Scitex application describe methods by which the photohardenable liquid is introduced into a vat by means of a pump or similar apparatus such that the new liquid level surface forms in one layer thickness over the previously exposed layers. Such methods have the aforementioned disadvantages of the Hull method except that the deflection of the layers during coating is reduced.
The patent issued to Fudim describes the use of a transmitting material to fix the surface of a photopolymer liquid to a desired shape, assumably flat, through which photopolymers of desired thickness are solidified. The transmitting material is usually rigid and either coated or inherently non-adherent to the solidified photopolymer. The methods described by Fudim do not address the problems inherent in separating such a transmitting material from a photopolymer formed in intimate contact with the surface of the transmitting material. Whereas the effects of chemical bonding may be reduced significantly by suitable coatings or inherently suitable films, the mechanical bonds along with hydrogen bonds, vacuum forces, and the like are still present and in some cases substantial enough to cause damage or distortion to the photopolymer during removal from the transmitting material surface.
The present invention provides a simple method and apparatus for forming a film through a transparent barrier in contact with a photohardenable composition in a manner that the film does not adhere to the barrier, due to thermal or photo- formation of a liquid layer between the transparent barrier and the photohardened film. It further provides a method and apparatus for fabricating integral three-dimensional objects from a multiplicity of cross sectional portions of the object, corresponding to photohardened portions of successive individual layers of a photohardenable composition.